The present invention relates to solar cells and in particular to solar cells having an anti-reflection coating which is applied to the solar cell prior to electroding.
Photovoltaic devices such as silicon solar cells promise a viable alternative to non-replenishable fossil fuel energy generation. Light energy (photons) incident on a solar cell's surface must enter and be absorbed within the cell to be converted to electrical energy. The efficiency of the solar cell is directly proportional to the amount of light absorbed by the cell.
Depending upon the particular cell fabrication process, the surface of the solar cell may be substantially reflective of light energy, reducing the solar cell's efficiency. Polished silicon, for example, may have a reflectivity of 40% in the spectral region of 0.35 and 1.2 microns. The problem is well recognized in the art and numerous solutions have been employed. A known effective technique for reducing the unwanted reflection is an anti-reflection layer in contact with the solar cell's surface. The anti-reflection (hereinafter A-R) layer is selected to coordinate its index of refraction, thickness and transmittance characteristics to surface reflection characteristics and the spectral region of interest. For silicon solar cells having a principal spectral absorbance between 0.35 and 1.2 microns, metal oxides such as tin oxide and titanium dioxide as well as magnesium fluoride are known anti-reflective coatings. Typically these layers are applied to the cell's surface after the cell fabrication has been completed, coating virtually the entire cell surface including the cell's electrodes. Applying the A-R coating after the completion of the cell restricts the temperature at which the A-R coating may be applied or treated. In contrast to the typical coating sequence, the present invention teaches an A-R coating method where an A-R layer precedes the electroding of the solar cell, avoiding the temperature limitations of the aforedescribed prior art process. The process is of further advantage in permitting electrical contact to the cell's electrode after the A-R coating, facilitating automated production of solar cells and solar panel assembly.
The art has generally recognized the advantage of exposing at least a portion of the solar cell's electrodes through the A-R coating. In U.S. Pat. No. 3,949,463, for example, Lindmayer et al teach a method for applying an A-R coating to a silicon solar cell where the A-R coating does not overcoat the cell's current collecting electrode. The technique is further exemplified in U.S. Pat. No. 3,904,453 where Revesz et al use photolithographic techniques in the formation of solar cell electrodes which are not overcoated with the cell's A-R coating.